1. Field of Invention
The present invention relates to a crank journal support portion structure of a horizontal opposed type engine and, especially, a crank journal support portion structure of a horizontal opposed type engine wherein an opening portion is provided in an area of crank journal support wall portions disposed between cylinders where the influence of stress concentration due to combustion load is small.
2. Description of Related Art
Generally, as for reciprocating engines whose cylinder disposition is of type other than a horizontally opposed type, such as straight-type or v-type engines, as shown in FIG. 4 and FIG. 5, a crank journal support wall portion 45 of a crank shaft 42 is disposed between respective cylinders 43, in a crank case 41 defined at the bottom of an engine cylinder block 40. Such engines are configured such that the crank shaft 42 is fit inside the crank case 41, by arranging a crank journal 44 via a bearing on a semi-circular journal bearing portion 46 formed at this crank journal support wall portion 45, and fixing with a bearing cap 47 from the engine bottom side.
In this case, a hole portion is often provided in respective crank journal support wall portion 45 penetrating in a thickness direction thereof such that respective cylinders can be in mutual ventilation through this hole portion. This hole portion is provided according to the following objects.
A first object is to solve engine pumping loss in the crank case.
Namely, during the piston vertical movement due to combustion in a certain cylinder, for example, in the explosion stroke, a piston is pushed down to the bottom of the cylinder by combustion energy generated by explosion in a combustion chamber. At this moment, air in the cylinder is compressed at the piston back side. In this case, this compressed air attenuates the kinetic energy of the descending piston, and energy transmitted to the crank via a connecting rod may be lost.
On the other hand, in the exhaust stroke, as the piston elevates in the cylinder while sucking air of the rear side of the piston, at the same time, a negative pressure is generated at the rear side of the piston and kinetic energy transmitted to the crank may be reduced.
Power loss caused by such piston movements is generally called "pumping loss" in the crank case, and a hole portion is provided in the respective crank journal support wall portion penetrating in the thickness direction thereof, in order to resolve such pumping loss in the crank case. It is configured such that respective cylinders can ventilate mutually through this hole portion.
Namely, for example, when a descending piston compresses air in the cylinder, power loss caused by the air compression generated at the piston back side can be reduced by discharging compressed air to adjacent other cylinders through the hole portion. Air led into the other adjacent cylinders can attenuate the negative pressure generated at the back side of the elevating piston.
As the result, for the multiple cylinder engine, such relation is created among a plurality of cylinders, allowing, consequently, to reduce generation of pumping loss in the crank case for the engine as a whole.
As another object of providing a hole portion in the crank journal support wall portion 45, it serves to evacuate blow-by gas leaking from piston ring gap to the crank case 41 side in the compression stroke and explosion stroke of the engine. Further, it contributes to reduction of the engine weight.
For these objects, the hole portion is often opened at respective crank journal support wall portion 45 penetrating in thickness direction thereof; however, for the horizontal opposed type engine, its conditions made it difficult to open the hole portion.
In other words, as for the horizontal opposed type engine, a pair of crank cases are opposed to each other and joined to hold the crank journal. The combustion loads in horizontal opposed type engines, therefore, act in mutual opposition along the piston displacement direction.
As a result, stresses due to the combustion load are also generated in mutually opposed portions of the horizontal opposed type engine. Further, these stresses are concentrated in both of the right and left sides of a joint portion of the crank case joined in opposition. As a result, when a ventilation hole portion is provided in the vicinity of the crank journal support hole portion formed in the crank case, the possibility of concentrating stresses generated due to the combustion load in the vicinity of the hole portion and lowering the strength of the engine in the vicinity of the hole portion exists in either of the right or the left crank case where the hole portion is provided.